All present oxygen production facilities extract oxygen from air. Air has the advantage of being free and available everywhere. One of the drawbacks is that because air is at atmospheric pressure, it contains a lot of water and CO2 at low partial pressure. And pressure drops in process cycles are energy expensive close to atmospheric pressure. It is the reason why most oil, chemical or petrochemical processes operate in the range of 10-40 bar. The pressure drops are less costly, heat exchange is easier, and the size of plants is reduced, drastically decreasing overall cost.
In the case of oxygen production, as air contains 80% nitrogen, a low pressure waste gas containing the nitrogen is normally produced. In case of cryogenic distillation, the cold heat contained in the waste nitrogen has to be recuperated through heat exchangers which are costly both in investment and related energy needs.
Some oxygen plants operate at higher than normal pressure with some means and additional investment to recover the energy lost in the waste nitrogen. FIG. 1 shows a basic power gas turbine arrangement in which an air compressor 1 sends air 3 at between 8 and 35 bar to a combustor 5 fed by fuel 6. The combustion gas 7 mixed with dilution air 4 forms mixture stream 8 which is expanded in gas turbine 9 having an inlet temperature between 900 and 1400.degree. C. and generates power. To achieve good combustion in the burner, a close to stoichiometric mixing is necessary to use fuel efficiently and produce minimum pollution. But in this case, combustion produces a hot gas at temperatures higher than 2000.degree. C., well above what any kind of hot turbine can accept. For this reason, quench type cooling takes place by mixing this very hot flue gas 7 with compressed dilution air 4 from the compressor at the same pressure as stream 3 but much lower temperature. The dilution air flow 4 is of the same order of magnitude as the combustion air flow 3.
Because this dilution air 4 does not participate in the combustion, oxygen is not necessary. So it is possible to extract the oxygen contained in the dilution air 4 as shown in FIG. 2. The air 4 is cooled, purified and distilled in separation unit 12 producing oxygen 10 and nitrogen 11. The nitrogen 11 is mixed with combustion gas 7.
Generally the separation unit used in a double column comprising a thermally linked high pressure column and low pressure column. However it is known to use a single column with a top condenser and a bottom reboiler for this purpose.
If the amount of nitrogen 11 is limited, it may alternatively be mixed with air stream 3 and sent to combustor 5 as described in U.S. Pat. No. 4,224,045. Another option is to send the nitrogen to be mixed with the fuel stream 6.